Mysterious coolant leak

[aps1231]

Just got my Boxster from the 60,000 miles service. It ran like a dream for a week, then I noticed the engine temperature going up abnormally. It was short one gallon of coolant. I filled it up, drove three miles to the shop, then the temperature started going up again. I stopped immediately and witnessed another gallon of coolant leaving the car, approximately next to the right rear wheel. I then got it towed to the shop.

They pressure tested it, let the engine run for ~45', no leak!! They say there's nothing wrong with it!?

Also, worth to mention, the Porsche dealer had to replace the overflow tank (cracked) and the cap one year ago. I know use a mechanic specialized in imports, after a few bad experiences with the dealer.

I noticed similar stories in the forum, but didn't have a good explanation. Does it sound like the overflow tank again (although pressure tested) or the water pump?

Thanks in advance for any tip you can give me.

aps